"Understand?" the doctor went on. "When that sculpture was made, Mercury was little nearer the sun than the earth is now!" The builder was hugely impressed. He asked, eagerly: "Then probably the people became as highly developed as we?" Van Emmon nodded approvingly, but the doctor opposed. "No; I think not, Jackson. Mercury never did have as much air as the earth, and consequently had much less oxygen. And the struggle for existence," he went on, watching to see if the geologist approved each point as he made it, "the struggle for life is, in the last analysis, a struggle for oxygen. "So I would say that life was a pretty strenuous proposition here, while it lasted. Perhaps they were--" He stopped, then added: "What I can't understand is, how did it happen that their affairs came to such an abrupt end? And why don't we see any--er--indications?" "Skeletons?" The architect shuddered. Next second, though, his face lit up with a thought. "I remember reading that electricity will decompose bone, in time." And then he shuddered again as his foot stirred that lifeless, impalpable dust. Was it possible? As they passed into the great house the first thing they noted was the floor, undivided, dust-covered, and bare, except for what had perhaps been rugs. The shape was the inevitable equilateral triangle; and here, with a certain magnificent disregard for precedent, the builders had done away with a ceiling entirely, and instead had sloped the three walls up till they met in a single point, a hundred feet overhead. The effect was massively simple. In one corner a section of the floor was elevated perhaps three feet above the rest, and directly back of this was a broad doorway, set in a short wall. The three advanced at once toward it. Here the electric torch came in very handy. It disclosed a poorly lighted stairway, very broad, unrailed, and preposterously steep. The steps were each over three feet high. "Difference in gravitation," said the doctor, in response to Jackson's questioning look. "Easy enough for the old-timers, perhaps." They struggled up the flight as best they could, reaching the top after over five minutes of climbing. Perhaps it was the reaction from this exertion; at all events each felt a distinct loss of confidence as, after regaining their wind, they again began to explore. Neither said anything about it to the others; but each noted a queer sense of foreboding, far more disquieting than either of them had felt when investigating anything else. It may have been due to the fact that, in their hurry, they had not stopped to eat.